1. Field of the Invention
The present invention relates to a method to detect and control detonation phenomena in an internal combustion engine.
2. Description of the Related Art
A spark ignition internal combustion engine includes a number of cylinders, each of which is provided with a piston which cyclically slides inside the cylinder and with a spark plug which is cyclically driven by an electronic control unit to make a spark between its electrodes and thus ignite the compressed gases inside the cylinder. The control unit comprises a memory storing a series of maps that provide the spark plug controlling values as a function of the current engine point. In particular, for each spark plug, the maps provide the value of the ignition advance, i.e. the value of the rotational movement between the ignition, namely the spark between the spark plug electrodes, and the top dead centre or TDC of the piston. If the value of the ignition advance is zero, then the ignition, i.e. the spark between the spark plug electrodes, takes place exactly in correspondence of the top dead centre or TDC of the piston.
The ignition advance values stored in the maps contained in the control unit are set during the engine tuning step, trying to ensure a good combustion under all possible operating conditions to have a good thermal efficiency of the engine and, at the same time, to guarantee the integrity of the engine, namely to avoid the presence of excessive detonation phenomena inside the cylinders. The detonation is an explosive-type combustion of part of the air-fuel mixture occurring before the mixture is reached by the flame front generated by the spark plug. The detonation creates a series of pressure waves that pass through the combustion chamber, violently striking against the metal walls. The detonation occurs when inside the chamber certain critical temperature and pressure values (which can quite considerably vary from engine to engine) are exceeded and, when this occurs at medium-low speed, it often causes a typical metal noise, clearly perceptible and known as “knocking”.
The detonation normally occurs when the ignition advance is excessive, when it has been used a fuel with a too low octane number (the antiknock power of the fuel is precisely indicated by its octane number) or in supercharged engines, when the supercharging pressure is too high. The combustion progress is influenced by many factors (among which the most important are the fuel characteristics, the temperature of the engine cylinder head, the spark plugs degradation), whose effect is substantially impossible to predict accurately.
For this reason it is necessary to detect the possible presence of an excessive detonation and, in case of an excessive detonation in a cylinder, the control unit must promptly react to avoid the recurrence of detonating phenomena also in combustion cycles subsequent to the combustion cycle in which the detonation has been detected.